Can You Use 2 Sim Cards On 1 Phone?

You can use multiple Sim cards on one Cell Phone. But you would either have to buy a cell phone with dual Sim pockets or a dual Sim card holder that you can insert in any cell phone. Either way she will be able to answer calls that are made on either of the Sim and can call from either of the Sims. What's better she can use Sims from two different service providers at the same time. The link below  features one such CECT 168, a dual Sim card phone, that will work for your needs:
Dual Sim phone

Yes you can use 2 sim cards on 1 phone, however its a handset feature. It is only applicable on those handset, who have this facility. Most important thing is you'll be able to receive incoming calls from both the numbers however you wont be able to make calls from both the numbers simultaneously. If you wanted to make calls from sim1.  Then you will have to switch to sim 1 or line 1. And when you wish to make calls from sim2, in that case you will have to switch to line 2 or sim 2. But you wont be able to make outgoing calls from both the sim cards at a time..

Yes you can. You can change the sim cards whenever you want. But, in some countries, some service providers sell the phone number with a cellular phone in a package. Some of those cell phones may be service provider restricted; specially produced for the provider. In other cases, a cellular phone may call with all sim cards..